The present invention relates generally to fuel injection pumps for internal combustion engines and relates more particularly to single plunger distributor type pumps for multi-cylinder diesel engines.
In the conventional diesel fuel injection pump, the fuel in the fuel distribution passages between the pump plunger and the nozzle valve seat is maintained at a relatively high residual pressure, for example 2,000 psi, by means of one or more check valves known as delivery valves, the purpose of which is to prevent cavitation within the passages when the pressure therewithin is suddenly dropped upon injection cutoff.
The presence of delivery valves in the injection lines is effective to maintain residual line pressure but is disadvantageous for a number of reasons. The presence of the delivery valve increases the dead volume of fuel between the pumping chamber and the injection line which must be pressurized during the plunger injection stroke before the nozzle is opened. The fuel flow rate through the pump passages, delivery lines and nozzle passages is accordingly relatively slow compared to a pump having a smaller dead volume as can be obtained by eliminating the delivery valve. Under peak pressure conditions, the increased flow rate obtainable with a smaller dead volume produces a larger positive reflected pressure wave at the nozzle with a resultant increase in nozzle flow rate. The effective nozzle pressure and hence the flow rate through the nozzle can accordingly be increased without increasing the pumping chamber pressure by reducing the dead fuel volume between the pumping chamber and nozzle valve seat.
The effective fuel pressure at the pump outlet is further increased by elimination of the delivery valve since a substantial pressure drop occurs across the valve. In tests of a typical pump, the pressure drop caused by the delivery valve circuit was found to be on the order of 700 psi.
Still another disadvantage of the conventional delivery valve is its interference with the proper functioning of certain types of leakless nozzles such as that illustrated and described in the present disclosure. In those types of leakless nozzles requiring a relatively low line pressure between injection intervals, some means such as a "leaking" delivery valve or its equivalent must be provided to lower injection line pressure to permit the necessary pressure relief of the nozzle valve spring chamber.
The present pump is eliminating the conventional delivery valve has gained the advantages enumerated above including a higher effective pressure at the nozzle and the ability to utilize the type of leakless nozzle requiring low residual line pressures. The elimination of the delivery valve, however, has raised the problem of cavitation in the fuel passages upon injection termination. During the rapid pressure drop, voids are apt to form which are subsequently collapsed by the reflected high pressure wave from the nozzle with a resultant erosion of the passage walls. It is the function of the present invention as described below to overcome this problem and to minimize cavitation erosion.